As you may recall, the parasympathetic nervous system has a cranial-sacral distribution where as the sympathetic nervous system has a thoraco-lumbar distribution. The sympathetic nervous system secretes the neurotransmitter norepinephrine in the post ganglionic sympathetic nerve fibers. Norepinephrine binds to end-organ adrenergic receptors to exert various effects on the various adrenergic receptors as seen below. It is worth while to remember these actions. Note: the neurotransmitter of the sympathetic preganglionic nerve fibers is acetylcholine.
Courtesy Clinical Anesthesiology third edition, Morgan, Mikhail et al
Uptake and metabolism of these neurotransmitters occur by several processes.
- diffusion away from the receptor over time
- degradation/metabolsim by monoamine oxidase and COMT(catechol-O-methyltransferase) within the post ganglionic nerve cells.
- reuptake by the postganglionic nerve endings.
Metabolism of secreted cathechols also occurs in the liver: see image courtesy of Clinical Anthesiology third edition, Morgan et al.
Understanding these concepts is key to understanding the actions of chatecholamines on the various receptors and end organs in which they reside. Various drugs that can increase catecholamine availability include:
- Monoamine oxidase inhibitors-by inhibiting the breakdown of Norepinephrine, more neurotransmitter is available for interactions with other drugs leading to hypertensive crisis and or serotonic syndrome.
- Tricyclic antidepressants- inhibit reuptake of Norepinephrine into nerve terminals, thus increasing the availability in the synaptic cleft.
- Too much circulating catecholamines over time leads to desensitization and hyporesponsiveness to further stimulation, as occurs in pheochromcytoma patients.
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